Clinical and Laboratory Correlates of Acute Respiratory Failure in Patients with COVID-19 Depending on the Values of Transcutaneous Saturation

With a traditional approach to treatment of hypoxemic respiratory failure, it is believed that SpO2 reduction below 88-90% during oxygen therapy requires emergency care including invasive mechanical ventilation. However, the manifestations of hypoxemic respiratory failure in COVID-19 patients have certain features that have led to the change in the traditional respiratory support procedure. The therapeutic goals of respiratory support in this category of patients require clarification.The objective: in patients with COVID-19, to study the relationship of transcutaneous saturation values with clinical indicators that characterize ARF, the state of acid-base balance and blood gas composition.Subjects and methods. A multicenter prospective observational study included 90 COVID-19 patients treated in ICU whose transcutaneous saturation (SpO2) values were below 93% despite treatment. Depending on the degree of impaired oxygenation, patients underwent oxygen therapy through a mask or nasal cannula, high-flow oxygenation or non-invasive ventilation, while it was not always possible to achieve the target values of oxygenation parameters. The patients were divided into the following groups: Group 1 ‒ SpO2 above 93%, Group 2 ‒ SpO2 within 93–90%, Group 3 ‒ SpO2 within 85–89%, Group 4‒ SpO2 within 80–84%, Group 5 ‒ SpO2 within 75–79%, and Group 6 – below 75%.Results. It was revealed that during ARF management by noninvasive methods, different values of transcutaneous saturation and corresponding changes in the acid-base balance (ABB) and blood gas composition were determined When transcutaneous saturation (SpO2) decreased to 85%, there was a corresponding moderate decrease in PaO2 while no metabolic changes occurred. As a rule, there were no obvious clinical signs of respiratory failure (silent hypoxia). In patients with SpO2 reduction down 80–85%, clinical signs of respiratory failure (dyspnea, tachypnea, agitation) and, as a rule, a moderate increase in PаCO2 with the development of respiratory acidosis and compensatory metabolic alkalosis were noted. When SpO2 decreased down to 75–79%, arterial hypoxemia was usually accompanied by moderate hypercapnia and the development of decompensated mixed acidosis and venous desaturation as well as increased lactate levels. With transcutaneous saturation going below 74%, these changes were even more pronounced and were observed in all patients of this group.Conclusion. The revealed changes are mostly consistent with generally accepted ideas about the relationship between values of transcutaneous saturation and blood gas composition and parameters of blood ABB in the case of ARF. Reduction of transcutaneous saturation down to 85% not accompanied by pronounced clinical signs of respiratory failure (dyspnea, tachypnea, agitation), development of acidosis and venous desaturation, and the elevated lactate level can be regarded as relatively safe.